The Celtic job is not easy. John Barnes should know that better than anyone.

It was George Bernard Shaw, in his play Man and Superman, who coined the infamous phrase that those who can’t do, teach. In short, if you cannot succeed in a profession because you don’t have the skill set, then you can at least teach that profession to others who wish to learn. It’s a derogatory statement, one that’s been applied to academics, literary critics, and numerous people in the sports commentariat — with a slight variation: those who can’t do can still criticise.

There’s a wonderful example in the life of the brilliantly talented but fatally flawed late Peter Bogdanovich. Bogdanovich was one of the golden boys of New Hollywood’s Director’s Era. He had made a splash with his first couple of films and was so arrogant and full of himself that he wouldn’t introduce himself to people, on the proviso that if you didn’t already know who he was, you weren’t worth knowing.

As his directorial career crumbled and his personal reputation suffered lamentably because of his attitude and notorious egotism, his standing in the business plummeted. But it gave him a new sense of self, and a new humility, so that whenever he was introduced to people afterwards, he would say, “Hello. I used to be Peter Bogdanovich.”

Bogdanovich went into writing about film and commenting on it rather than making his own movies for many, many years.

But he was an outstanding writer on film, an outstanding critic, an outstanding advocate for other filmmakers. The depth of his knowledge and passion for movies of all shapes, sizes, and genres is testified to in the number of books he wrote, the number of awards he won as a writer, and the fame he built for himself as a supremely knowledgeable cinephile. And eventually, Bogdanovich returned to directing and made some pretty good films towards the end of his career.

Turns out that Bogdanovich could both do and teach — that he could be creative and a critic at the same time. John Barnes should stick to being a critic because, sure as hell, he was no football manager.

It’s Barnes’ own lack of self-awareness that makes his recent comments about Brendan Rodgers so hilarious and stupid. Imagine someone like him talking about how easy the Celtic job is, and about how a manager succeeding in that job is still some way short of being elite. Imagine criticising Rodgers for returning to the club and proving again that he is one of the best in the business.

I mean, there may be people in the commentariat who are entitled to do that — those who both can and teach, those who’ve achieved enough that they can criticise. They’re very rare in the football commentary world, of course, because if you’re good enough, you’re still out there on the touchline prowling around, not stuck in the commentary box watching the action.

But Barnes is not in that small number. Barnes was a failure, not just at Celtic but in his managerial career as a whole.

And the thing is, he wasn’t just a failure — he was a terrible failure.

It wasn’t just that he lacked a couple of the skills necessary to manage a football team; he lacked any number of those skills, including the ability to manage a dressing room, which is one of the toughest jobs in the business — and one that, with some respect to him, greater men have failed at.

But when one of your star players throws his boots in the bin, that tells you everything you need to know about the respect people in the dressing room have for you.

We can’t know the ins and outs of everything that happened at Celtic Park under John Barnes. But when he later, years after the sacking, dismissed his failure there not as his own fault but as a result of racism — one of the stupidest allegations ever levelled at Celtic or any other club — he showed the complete lack of self-awareness that’s necessary to honestly and accurately assess where you were and how you got there.

Bogdanovich at least knew that he had been a pure fool in the early part of his career and that he’d made a lot of enemies while he was riding high and thought he’d be on top forever.

Barnes doesn’t have a shred of that. Barnes doesn’t know that he failed because he’s a loser. He doesn’t know that he failed because he’s just not good enough. He probably does believe that there was some sort of malignant intent behind it all, a desire to thwart him because of the colour of his skin — it’s ridiculous.

The truth is that Barnes should have been grateful to Celtic forevermore. Because Barnes came to this club with no coaching experience at all, and Celtic appointed him with no reason to do so whatsoever except on Kenny Daglish’s say-so. We offered him a chance at a stage in his career where nothing justified it but blind faith and had he made more of it then he’d owe it all to us.

But in the end there was nothing to back up his self-belief that he could translate his playing skill to management — although the two are so grossly different that even thinking one makes you qualified for the other should be grounds for disbarment from ever sitting in a dugout.

Barnes was not the first to suffer from that particular delusion, and nor will he be the last. I’m convinced that the guy sitting in the Ibrox dugout is just as delusional and just as stupid for believing that being a player means you’re qualified to be a coach. And I believe that, in the fullness of time, if he stays in post, we’ll all see Ferguson’s mad belief shatter as it did several times before he got the Ibrox gig — and his failures at the other clubs where he’s tried to prove the theory.

But Ferguson at least had those chances. Ferguson at least had enough about him that other clubs gave him that opportunity.

Barnes had nothing. Barnes didn’t get another major job after leaving Celtic. He was lucky to get a handful of minor jobs after leaving Celtic; he had a brief spell as head of the Jamaican national football team, and twelve games at Tranmere Rovers where he lost eight of them before they bagged him.

I don’t hate Barnes. I have no problem with John Barnes as a person. He’s done an awful lot for charity for a start, where he’s widely regarded as one of the most prominent voices across various charitable endeavours. But he proves the old adage that those who can’t do, teach — because, of course, he now runs coaching courses. And like Bogdanovich, he also branched out into commentary and is very good at it, a little bit like Neil Lennon, about whom I might write more later on.

But I know this — he’s no manager, and he is not remotely qualified to critique Brendan Rodgers and the choices Rodgers has made in his managerial career. A managerial career which is everything John Barnes’ was not.

And I think his intervention here has only made him look ridiculous. It’s one of the most embarrassing statements ever to come from someone who has failed as completely in management as he has. And when he has the sense to look back on it later, he should be mortified by it.